In a very clean and dry bowl, whip egg whites to stiff peaks. Bring sugar and water to a boil. Occasionally use a pastry brush and cold water to brush down any crystals that form on the side of the pan. Boil for seven minutes over medium heat. Pour sugar mixture over egg whites, beat until cool to the touch, about ten minutes. It may be helpful to place this bowl under a bowl with a little cold water to speed up the cooling process. Especially if your kitchen is particularly hot. Slowly add the butter. Your buttcream may look a little runny or thin at this stage, but thats okay. Once butter is fully incorporated add vanilla. Beat until light and fluffy. It may look curdled, but this is also okay. Keep on mixing!
This icing can be refrigerated for weeks, or even frozen for months. To get it back to be nice and creamy after being in the fridge or freezer, just mix it again, and it will return to its' creamy texture.